Sound attenuating gas conduit



1964 E. LUDLOW ETAL SOUND ATTENUATING GAS coNnuI'r 2 Sheets-Sheet 1Filed Feb. 13, 1961 i'illl hllll iP "6.0.:

INVENTORS. {UNI/N0 Luoww AND BYBE/VJ'AM/N M I ,ewuv w OOQOOOGO A TTORNEys.

E. LUDLOW ETAL SOUND ATTENUATING GAS CONDUIT Jan. 28, 1964 2Sheets-Sheet 2 Filed Feb. 13, 1961 INVENTORS. ZZJMUND L001. ow Am: BY35mm M/IV H. Imam United States Patent 3,119,459 SOUND ATTENUATING GASCONDUIT Edmund Ludlow and Benjamin H. Irwin, Columbus, Ind, assignors toArvin Industries, Inc., Columbus, Ind., a corporation of Indiana FiledFeb. 13, 1961, Ser. No. 88,977 Claims. (Cl. 181-59) This inventionrelates to a sound attenuating gas conduit for conveying, andattenuating the noise level of, a moving gas stream, and which is welladapted for use with an internal combustion engine for conveying theexhaust gases therefrom and silencing the noise level of said exhaustgases.

It is an object of our invention to provide such a sound attenuating gasconduit which will meet limited space requirements, which can be easilymanufactured largely from inexpensive metal-tubing, and which can be oflightweight construction with its weight substantially uniformlydistributed along its length. It is a further object of our invention toprovide such a sound attenuating gas conduit which can be made to effectsound attenuation over a wide range of frequencies, which may be tunedto attenuate undesired frequencies, and which will remain substantiallyin tune with said frequencies irrespective of temperature changes of thegas stream in which'the sound waves are carried. It is still a furtherobject of our invention to provide such a sound attenuating gas conduitwhich will be less susceptible to certain types of corrosion thanconventional gas-silencing systems, and which may employ replaceablesound attenuating units.

it is a special object of our invention to provide a sound attenuatinggas conduit for the exhaust gas stream of an automotive vehicle, whichwill eliminate the need for the bulky, expensive, and troublesomemuffiers which are required in conventional automotive exhaust silencingsystems.

We have discovered that it is possible to attenuate the noise level ofindividual sound frequencies by suitably located tuned elements of muchsmaller physical size than the structures required in prior muffler-typesystems, and that by the use of a combination of such elements we canprovide a complete and practical automotive exhaust silencing systemwithin the configuration of a simple pipe extending from the usualautomotive engine to the usual point of discharge of the exhaust gases.

Our silencing elements may be used individually or in combination withother silencing means, and, in such applications as automotive exhaustsilencing, our invention contemplates a system embodying a number ofsilencing elements in interrelated arrangement.

In accordance with our invention as applied to an automotive exhaustsystem, the exhaust manifold is connected to a pipe to convey theexhaust gases to the desired discharge point, as at the rear of thevehicle. Acoustically, such pipe, usually with at least part of theexhaust manifold, forms a conduit in which the exhaust sound producesstanding wave pressure patterns, wherein each of the several harmoniccomponents of the standing waves has one or more distinct pressurepoints, that is, points of maximum sound pressure, at particularlocations along the conduit. In accordance with our invention, soundattenuating elements are placed at or adjacent to these pressure points,and are respectively made to attenuate the particular harmonicfrequencies or bands of frequencies having pressure points at oradjacent such locations.

In order to maintain effective attenuation under the varying temperatureconditions and gradients which will occur, the silencing elements arearranged in direct and close thermal coupling relation with the gasstream, as

I 3,119,459 Patented Jan. '28, 1964 within and surrounded by the gasstream. The silencing elements may include resonance cavities or volumesand one or more throats for acoustically coupling the volumes with thegas stream. These thermal and acoustical coupling relationships with thegas stream are desirably made maximal as by disposing the volumes andthroats within, and axially symmetrical with, the gas stream.

Other forms and locations of silencing elements may also be used. Forexample, units for suppressing relatively wide bands of high sound wavefrequencies may be placed at pointsnot occupied by specifically tunedelements.

Other objects and features of our invention will become apparent fromthe more detailed description which follows and from the accompanyingdrawings, in which:

FIG. 1 is a fragmentary isometric view partially in section showing asound attenuating exhaust system embodying our invention;

FIG. 2 is an enlarged fragmentary longitudinal section of the exhaustsystem shown in FIG. 1, but with the outer pipe shown as interfittingpipe sections;

FIG. 3 is an enlarged vertical section taken on the line 33 of FIG. 2;

FIG. 4 is an enlarged fragmentary longitudinal section of anotherportion of the exhaust system shown in FIG. 1;

FIG. 5 is a fragmentary longitudinal section showing a different form ofsilencing element;

FIG. 6 is an enlarged vertical section taken on the line 66 Otf FIG. 5;

FIG. 7 is a fragmentary longitudinal section showing another form ofsilencing element;

FIG. 8 is an enlarged vertical section taken on the line 8-8 of FIG. 7;and

FIG. 9 is a vertical section of a modified embodiment of our invention.

Our invention is particularly well adapted for use with an internalcombustion engine in an automobile to silence the exhaust gasesemanating from said engine and to convey them to a suitable dischargepoint. In such usage, it completely replaces a conventional exhaustsystem in which all of the silencing effect is lumped in specificlocations determined by the structural requirements of the vehicle, asin a muffler connected between an exhaust pipe joined to the exhaustmanifold of the engine and a tail pipe leading from the mufi ler to agas discharge point. Such a muffier conventionally comprises an outershell having an elliptically or circularly shaped cross-section manytimes larger than the cross-section of the exhaust and tail pipes, andhas a relatively small number of large resonators, each adapted toattenuate a wide band of sound frequencies in the exhaust gases passingthrough the system. In such muffiers, there are normally provided twolarge resonator chambers respectively tuned to attenuate the harmonicfrequencies of the exhaust and tail pipes. All of the resonator chambersare baflled one from another, and are arranged within the mufiler shellin staggered patterns which, combined with the large size of theresonator chambers for the exhaust and tail pipes, result in the mufilerbeing large and difficult to mount in the limited space available on theunderside of an automotive vehicle.

Our invention, however, avoids the need for such large mufliers inspecific locations by the employment of a series of small silencingelements in in-line relationship Within the gas stream in a pipeextending from the manifold to the gas discharge point. The differentsilencing elements may be designed to attenuate different and/oroverlapping bands of wave frequencies, and may be located in the pipewith respect to the harmonic characteristics of the pipe so that theywill effect sound attenuation without the use of any large bulkyresonator chambers,

as are required in a conventional mufller-type system. Our inventionthus provides an exhaust system which has a substantially smaller sizeand a lighter weight than a conventional muffler-type system, whilestill effecting at least the same degree of attenuation as aconventional muflleratype system.

In muffler-type silencing systems, the muffier silencers used provide inthe flow passage of the system a substantial, abrupt enlargement of thecross-sectional area of the passage, with an abrupt increase in area atthe upstream end of the silencing structure and an-abrupt decrease inarea at the downstream end. Among other things, this produces a velocitydecrease and an abrupt gas expansion which reduces the temperature ofthe gas and tends to condense exhaust vapors in position to be trappedand corrode the muffler structure. In contrast to this, our inventionavoids such enlargement of the gas-flow passage, and desirably uses anouter pipe of substantially uniform cross-section with the silencingelements; disposed Wholly within such cross-section, which produces areduction instead of an enlargement of the flow passage area at thelocation of a silencing element. While we avoid the abrupt enlargementof prior systems, and conveniently have reductions instead, we may formthe structure to give a flow passage of generally uniform area.

In the operation of a conventional internal combustion engine in anautomobile, the combustion of fuel within the cylinders produces asubstantial volume of hot exhaust gases which are exhausted withsubstantial noise into the exhaust manifolds mounted on the engine incommunication with the cylinder exhaust ports. The frequencies of thesound waves in such exhaust gases extend over a wide range, such as fromabout 30 cycles/sec. to about 5,000 cycles/see, with the lowerfrequencies largely representing the fundamental and lower harmonicsdetermined by the length of the exhaust conduit. In many exhaust systemsit is the lower range of frequencies, i.e., frequencies below 200cycles/sec. that are the most difficult to attenuate and produce themost objectionable noises, especially since it is in this low frequencyrange that the firing frequencies of the engine coincide with andaugment the natural resonant frequencies of the exhaust system itself.In many conventional mufilers these low frequencies are not adequatelyattenuated because the large size of the mufilers prevents them frombeing positioned on the underside of a vehicle in the proper positionswith respect to the exhaust system to act upon and attenuate these lowfrequencies.

Our invention is adapted to attenuate the noise level of the exhaustgases over a wide range of frequencies, including the troublesomefrequencies below 200 cycles/ sec., by passing said gases through anexhaust conduit having a series of silencing elements mounted within italong its length. The silencing elements may take several forms. Forexample, one form may be used to attenuate frequencies up to 300cycles/see, another form to attenuate frequencies in the range of 100cycles/see, to 700 cycles/sec, another form to attenuate frequencies inthe range of 300 cycles/ sec. to 1,500 cycles/see, and still anotherform to attenuate those frequencies above 1,500 cycles/sec.-

While silencing elements in accordance with our invention may be usedalone to effect attenuation of the exhaust gas noises, they may be usedin combination with conventional mufiiers, or may be incorporated withinotherwise conventional mufflers as acoustical muffler components, orused in combination with acoustical liners lining axial sections of theconduit.

The embodiment shown in FIG. 1 comprises a pipe 10 adapted to beconnected at one end to an exhaust manifold 12 by a conventionalmounting flange 14, with its opposite end open to the atmosphere.Conveniently, the pipe 10 may have the same outer diameter as theexhaust and tail pipes used in conventional exhaust systems. Forexample, it may have a diameter of about one and three-quarters to twoand one-half inches, the diameters normally used in conventional exhaustpipes and tail pipes on automobiles; but it may have a larger diameter,say as large as four inches, the diameter of conventional exhaust andtail pipes in trucks, buses, and other large vehicles. While the pipe 10may be a unitary length, it may be formed from a plurality of shortinterconnected lengths of pipe, as indicated in FIG. 2, to facilitatethe installation and replacement of the silencing elements.

In the exhaust system illustrated in FIG. 1, a plurality of lowfrequency silencing elements 16 and high frequency silencing elements 18are mounted in the pipe 10 in the path of the gas stream moving throughsaid pipe. As shown in FIG. 2, each of the elements 16 comprises anelongated wall, conveniently in the form of a length of metal-tubing 20,closed at its opposite ends as by inturned flanges forming end walls 22.A baflle plate 24 is mounted within the tubing 20 intermediate its endsto divide the tubing 20 into a pair of elongated cavities or resonatorvolumes 26. Each of the volumes 26 has associated with it a resonatorthroat-forming tube 28 having one of its ends open to its respectivevolume 26 and its opposite end open to the gas-flow passage formed bythe pipe ll). In this manner, each of the volumes 26 is directly coupledby means of its throat 28 with the gas stream moving through the pipe 10so that the resonators formed by said throats and volumes will attenuatethe noise level of sound waves in the exhaust gases.

In the embodiment of FIGS. 1 and 2, the elements 16 are supported in thepipe 10 by brackets 30, conveniently in the form of sheet-metalstampings. The brackets 30 are rigidly secured to each of the soundattenuating elements 16 and to the wall of the pipe 10, and support theelement 16 in spaced relation to the wall of the pipe 10. Thus, theelements 16 are maintained out of contact with the wall of the pipe 10and out of contact with any corrosive liquids condensed out of theexhaust gases onto the pipe wall, said liquids thus being free to drainout of the pipe. Further, the brackets 30 support the elements directlyin the gas stream so that said elements are thermally coupled to the gasstream and will assume the temperature changes of said gas stream.

In order that the system of resonators formed by the volumes 26 andthroats 28 will attenuate a substantial range of sound wave frequenciesin the exhaust gases, it is necessary that individual resonators betuned to given frequencies or frequency ranges at least approximatelycoordinated with the fundamental resonant frequency of the exhaustconduit. Such tuning may be effected by adjusting the conductivity ofthe resonator throat with respect to the size or capacity of theresonator volume. For resonators with tubular throats, the formula forcalculating the tuning may be represented by the formula:

C C 21r V where f is the frequency to which the resonator is to betuned, C is the speed of sound in inches per second at the temperatureof the medium, V is the capacity of the resonator volume, and C is theconductivity of the resonator throat calculated from the formula:

While each resonator attenuates to the maximum degree the particularfrequency to which it is tuned, it will, of course, attenuate to alesser extent a limited band of frequencies on either side of thatparticular frequency, and will effect some attenuation of substantiallyall frequencies.

As will be understood from well known principles of acoustics, thefundamental resonant frequency of the exhaust conduit, with which thefrequncies of the resonators are to be coordinated, depends upon thespeed of sound. As shown by the first formula set forth above, thefrequency of a resonator likewise depends upon the speed of sound. Sincethe speed of sound varies with temperature, differences between thetemperatures of the resonators and the exhaust gases will interfere withthe coordination necessary for the resonators to achieve their maximumattenuation.

These changes in the speed of sound resulting from changes in thetemperature of the medium in which the sound waves are carried will alsocause the frequencies of the sound waves to change, the degree offrequency change depending upon the temperatures and frequenciesinvolved. For example, we have found that in the temperature range of 72F. to 500 F., each 100 F. change in temperature will produce thefollowing frequency changes in a conduit whose fundamental wavefrequency is 37.7 cycles/sec: its fundamental wave frequency of 37.7cycles/sec. will change 3 cycles/see; its second harmonic of 75.4cycles/sec. will change 6 cycles/see; its third harmonic of 113.1cycles/sec. will change 9 cycles/sec; etc. Thus, in the normal operationof an automobile where the exhaust gases may range from a temperature ofabout 200 F. when the engine is cold to a temperature of about 1,700 F.when the engine is hot, the harmonic frequencies of the pipe 10 aresubject to wide changes in the lower frequency ranges. In our system, weplace the silencing elements directly in the gas stream so that theresonators are subjected to the same temperature changes as the gasstream at the points where the resonators are located in said gasstream. This maintains the temperature difference between the resonatorsand the gas stream at a minimum, irrespective of gas stream temperaturechanges, and the resonators will thus remain coordinated with theresonant harmonic pipe frequencies which they are to attenuate.

Preferably, the resonators in the silencing elements 16 are respectivelytuned to attenuate the objectionable harmonics in the gases in theconduit. Each harmonic will have specifically located maximumsound-pressure points along the length of the conduit, the number ofsuch pressure points and their location being a function of theparticular harmonic involved. For example, the second overtone (thirdharmonic) will have three maximum pressure points along the conduitwhich will occur at points spaced from either end of the conduit bydistances of onesixth, one-half, and five-sixths of the conduit-length.Each of the resonators will attenuate to the maximum degree theparticular harmonic for which it is tuned if its throat opening iscoupled into the gas stream at any one of the points of maximum pressureof the harmonic for which it is tuned. While the resonators will effectmaximum attenuation if their throats are located precisely at theirmaximum pressure points, they will, of course, still operate at highattenuating efficiencies if their throats are located adjacent suchpressure points. For example, we have found that a resonator willoperate at not less than 90% efficiency if its throat opening is placedat any point within a distance from the true maximum pressure pointequal to one-twentieth of the length of the sound wave producing thepressure point.

In general, such maximum pressure points are spaced from an end of theconduit by fractions L of the conduitlength according to the formula:

2m 1 L- 2n where n is the harmonic number for which the resonator istuned, and m is every integer between and including 1 and n. Thus, if aparticular resonator is tuned to attenuate the second harmonic frequencyof the gases in the conduit, the above formula may be employed todetermine that the throat opening for the resonator should be spacedfrom one end of the conduit by a distance equal to one-fourth orthree-fourths of the length of the conduit.

It is to be understood that instead of a single resonator, two or moreresonators may be employed to attenuate a particular harmonic; indeed,in certain cases, substantially improved results may be obtained byusing the same total resonator capacity in two or more resonators ratherthan in a single resonator. If two resonators are employed to attenuatea particular harmonic, they should be spaced along the conduit either sothat their throats open to the conduit adjacent to the appropriatepressure points, or mounted in the conduit so that the throats of bothopen to the gas stream adjacent one of such pressure points.

For ease of construction, each of the silencing elements 16 is mountedonly in a straight section of the outer pipe 10, but the maximumpressure point of a harmonic frequency for which its resonator is tunedmay be in a curved section of the pipe. To overcome this difficulty andto permit the resonator to operate at its maximum efliciency, thesilencing element may be constructed with the resonator throatprojecting outwardly therefrom so that with the element mounted adjacentthe curved section, the throat will project into said curved section todispose its open end at the desired maximum pressure point location, asindicated at 27 in FIG. 1.

The relative positioning of the elements 16 with respect to each otheralso influences the action of the resonators with respect to thefrequencies which they attenuate. We have found that by positioning apair of elements 16 closely adjacent each other as in the relationshipshown in FIG. 7, the space between the opposed end walls 22 forms acommon annular extension of the throats 28. This increases the effectivelength of the throats 28 with the result that the opposed resonatorsattenuate lower frequencies than they would attenuate if their walls 22are spaced relatively farther apart. Of course, if the resonators arespaced too close together, the space between their opposed end wallswill be insufficient for the necessary acoustic coupling and willsubstantially reduce the attenuation efiiciencies of the resonators.Conversely, if the opposed end walls are spaced substantially apart,there will be no mutual throat action between the resonators and theywill act independently of each other.

The silencing element 16 shown in FIG. 2 with the elongated tubularthroats 28 is particularly well adapted for attenuating the troublesomesound wave frequencies "elow 300 cycles/sec. To attenuate thosefrequencies occurring above 1,500 cycles/sec, the silencing element 18is employed. As shown in FIG. 4, the element 18 comprises an elongatedtube 34, conveniently a length of metal-tubing, closed at each of itsends, and supported on a bracket 36 mounted in the pipe 10 to thusdispose the element 13 directly in the path of the gas stream movingthrough the conduit. The cavity within the tube 34- contains a porousfibrous material 38, such as asbestos fibers, stainless steel wool, orthe like. This material may be in the form of a porous wadding fillingthe cavity, a hollow sleeve lining the cavity walls, or any othersuitable configuration.

A plurality of perforations 39 are formed in the tube 34 to render it atleast 30% open, and thus dispose the cavity containing the porousmaterial 38 in operative acoustical communication with the sound wavesin the gas stream. The perforations 39 correspond to the throats 28shown in FIG. 2 in that they operatively interconnect the cavity of thetube 34 with the gas stream. However, because of their negligiblelength, the conductivity of 7 the perforations 39 is extremely high tothus cause the element 18 to preferentially attenuate the extremely highfrequency sound waves above 1,500 cycles/sec.

The type of silencing element shown in FIG. 4 will attenuate broad bandsof high frequency sound waves having large numbers of relatively closelyspaced maximum pressure points. Therefore, the axial positioning ofthese elements along the conduit is less critical than the placement ofsilencing elements for attenuating the lOWer frequency sound waves, andthe type of element shown in FIG. 4 may thus be disposed immediatelyadjacent the discharge end of the conduit downstream from the silencingelements for the lower frequency sound waves. This leaves the otherparts of the conduit free for mounting the lower frequency silencingelements at the maximum pressure points of the frequencies for whichthey are tuned.

Another form of silencing element in accordance with our invention isshown in FIGS. and 6, and comprises an outer pipe 40 having suspendedwithin it one or more silencing elements, each comprising an elongatedtube 42 closed at its ends and mounted on a channeled bracket 44 mountedin the pipe 40 to thus suspend the tube 42 in the gas stream in spacedrelation to the pipe 40. Conveniently, a plurality of baffle plates 45are mounted within the tube 42 to divide it in a plurality of axiallyspaced resonator volumes 46.

To operatively interconnect each of the volumes 46 with the gas stream,a plurality of throat-forming openings 48 are formed in the tube 42around its circumference within the axial extent of each of saidvolumes. Desirably, the openings are formed by shearing the tube 42 toprovide tongues 49 bent to project from the tube wall. In this manner,the openings 48 with their adjacent tongues 49 constitute resonatorthroats which, because of their sheared construction, have aconductivity such that the silencing element will attenuate sound wavefrequencies in the range of 300 cycles/ sec. to 1,500 cycles/ sec.

Still another form of silencing element in accordance with our inventionis shown in FIGS. 7 and 8, in which there is an outer pipe 52 formingthe gas-flow passage and having suspended within it pairs of silencingelements 54. As shown, each of the elements 54 comprises an elongatedtube 56 having one of its ends closed as at 58, and its opposite endprovided with one or more perforations 60 which constitutethroat-forming means for the resonator volume formed by the tube 56.Each of the elements 54 is mounted on a bracket 62 mounted in the pipe52 to thus dispose said elements in spaced relation to the pipe 52.

As previously described in connection with the silencing elements 16,the elements 54 may be arranged in opposed pairs with their perforatedend faces immediately adjacent each other. In such a closely spacedrelationship, the space between the opposed end faces forms a commonextension of the throat-forming means in the two resonators to increasethe effective length of said means and cause the elements 54 to act upona lower range of sound wave frequencies than the frequencies for whichthey are individually tuned. When the elements 54 are disposed in theiropposed relationship, they will attenuate sound wave frequencies in therange of 100 cycles/sec. to 700 cycles/sec, with their preferentialfrequency response in said range depending upon the size of the volumesof the tubes 56, the size and number of their perforations 6t), and thespacing between their opposed end faces. If it is desired to modify theeffective throat length of only one of the silencing elements, the pairof elements are mounted in alignment with the perforated end face of oneof said elements being disposed adjacent the nonperforated end face ofthe other.

It is to be understood, of course, that either of the elements 54 may beemployed without the use of the other and/or the throat-formingperforations in said elements may be disposed around the side walls ofsaid elements instead of in their end faces. However, the degree towhich the elements 54 are perforated will normally be very small incomparison with the large degree of perfora' tion present in thesilencing element 18 shown in FIGS. 1 and 4.

In all of the modifications shown, our silencing elements are carriedwithin an outer pipe forming the gas-flow passage whereby the gasesmoving through said pipe will have to pass through the space betweensaid pipe and the silencing elements. The pipe forming said passage hasa generally uniform cross-section with no substantial, abrupt changes inits diameter adjacent said silencing elements, and the averagecross-sectional area of the gas-flow passage bctween said elements andthe pipe within the axial extents of said elements is smaller than theaverage cross* sectional area of the gas-flow passage in the openportions of the pipe outside the extent of the silencing elements.However, the cross-sectional area of the silencing elements is in therange of from about 25% to about 75% of the cross-sectional area of thepipe within the axial C71LHt of said elements so that said elements willeffect the necessary degree of attenuation but will not unduly obstructgas flow through the pipe and create excessive back pressures.

As will be apparent, by employing several of the illustrated types ofsilencing elements in combination, a wide range of sound wavefrequencies may be attenuated, For example, elements of the type shownin FIGS. 2 and 4 may be employed to attenuate extremely low and highranges of frequencies, respectively, and elements of the type shown inFIGS. 5 and 7 may be used in combination therewith to attenuate theintermediate range frequencies. It is also possible to combine severaltypes of elements in a single combination structure carried within theouter pipe. For example, elements of the type shown in FIGS. 2, 4, and 5may be incorporated in an axially spaced relationship in the same lengthof tubing, and that length of tubing mounted in an outer pipe.

As shown in the drawings, the silencing elements are located in the pathof the gas stream movement. This permits our silencing elements to beintimately acoustically and thermally coupled to said gas stream so thatsaid elements will not only achieve their maximum acoustic efficienciesthrough such direct coupling, but in addition, their resonantfrequencies will adjust directly with the frequency changes in the gasstream due to temperature changes. Further, such suspension positionsthe silencing elements out of contact with the outer pipe to reducetheir susceptibility to corrosion. However, should such corrosion occur,the silencing elements may be easily replaced by simply removing thecorroded element and replacing it with a new element. As previouslydescribed, such replacement may be facilitated by forming the outer pipefrom a plurality of short interfitting sections of pipe so that thedefective silencing element and its respective pipe section may bereplaced as a unit.

While we have shown our silencing elements as disposed concentricallywithin an outer pipe on brackets, it is to be understood that suchbrackets may be omitted and the elements may contact the outer pipealong one or more lines of contact. To this end, the elements may besuspended from one line of contact along the wall of the outer pipe, orthe pipe and elements may have different cross-sectional configurationswhich permit the elements to contact, and be supported from, the pipealong two or more lines of contact. An example of the latter situationis shown in FIG. 9, in which the outer pipe indicated at it? isflattened into an eliptical cross-section, and the silencing elementindicated at 16" abuts it along two lines of contact and is securedthereto by welds '13. However, as in the embodiments of our invention,there are no substantial, abrupt changes made in the generalcrosssection of the pipe. Where the silencing elements are in linecontact with the pipe, the means supporting the elements therein maycomprise welded interconnections between the pipe and silencing elementsalong their lines of contact. However, even where the silencingelementscontact the pipe, the major portions of the walls of said elements aredisposed wholly out of contact with the outer pipe to provide a highdegree of coupling between the elements and the gas stream, and to causethe gases to move through the space between the elements and the p1pe.

For purposes of simplicity of description, we have only described ourinvention for use in an exhaust system for an engine. However, it may,of course, also be used on the intake side of an internal combustionengine for transporting and silencing the gas, intake flow to theengine, or for many other silencing applications.

We claim as our invention:

1. In a sound attenuating gas conduit for conveying, and attenuating thenoise level of, a moving gas stream, an open-ended pipe free fromsubstantial abrupt changes in its diameter forming a gas-flow passagehaving unrestricted flow at its ends, at least one silencing elementmounted within said pipe inwardly from the ends thereof and comprisinganelongated enclosed wall forming a sound attenuating cavity, andthroat-forming means in direct communication with the interior of saidpipe operatively interconnecting said cavity and gas-flow passagewhereby said element will silence the gases moving through said passage,the cross-sectional area of the gas passage within the axial extent ofsaid element being smaller than the average cross-sectional area of thegas passage outside the axial extent of said element, said silencingelement being mounted within said pipe in said gas-flow passage on abracket secured to said pipe to support said silencing element whollyout of contact with said pipe whereby said gas stream must pass throughthe space between said element and pipe during its movement through theconduit.

2. In a sound attenuating gas conduit for conveying, and attenuating thenoise level of, a moving gas stream, an open-ended pipe free fromsubstantial abrupt changes in its diameter forming a gas-flow passagehaving unrestricted flow at its ends, at least one silencing elementmounted within said pipe inwardly from the ends thereof and comprisingan elongated enclosed Wall forming a sound attenuating cavity, andthroat-forming means in direct communication with the interior of saidpipe operatively interconnecting said cavityand gas-flow passage wherebysaid element will silence the gases moving through said passage, saidelement having a cross-sectional area in a range of from about 25% toabout 75% of the crosasectional area of the pipe within the axial extentof said silencing element with at least substantially all of said wallin spaced relation to said pipe whereby said gas stream must passthrough the space between said element and pipe during its movementthrough the conduit.

3. The invention as set forth in claim 2 in which said wall comprises afirst length of tubing having enclosed ends, and said throat-formingmeans comprises a second length of tubing mounted on said first lengthof tubing, said second length of tubing having a smaller crosssectionalarea than said first length of tubing and having one of its ends open tosaid gas-flow passage and its opposite end open to the cavity formed bysaid first length of tubing.

4. The invention as set forth in claim 2 in which said wall comprises alength of tubing having enclosed ends, and said throat-forming meanscomprises a plurality of axially and circumferentially spaced openingsformed in said tubing, the portions of the tubing defining the edges ofsaid openings being deformed out of the general plane of said tubing.

5. The invention as set forth in claim 2 in which said wall comprises alength of tubing having enclosed ends, and said tubing is provided withone or more perforations comprising said throat-forming means.

6. The invention as set forth in claim 2 in which said wall comprises alength of tubing having enclosed ends and containing a porous, fibroussound-absorbing material, and said throat-forming means comprises aplurality of perforations formed in said tubing to render the same atleast 30% open.

7. The invention as set forth in claim 6 in which said sound-absorbingmaterial is in the form of a porous wadding filling said tubing.

8. The invention as set forth in claim 2 in which said Wall comprises anelongated length of tubing having enclosed ends and provided with aplurality of axially spaced bafile plates mounted on its interior todivide said cavity into a plurality of cavities, and said throat-formingmeans forms a plurality of passages each of which is open to saidgas-flow passage and one of said plurality of cavities.

9. The invention as set forth in claim 2 in which said wall abuts theinner face of said pipe along one or more lines of contact, and saidelement and pipe are rigidly secured together along at least one of saidlines of contact.

10. In a sound attenuating gas conduit for conveying, and attenuatingthe noise level of, a moving gas stream, a pipe forming a gas-flowpassage for said gas stream, a plurality of silencing elements mountedwithin said gas stream, each of said silencing elements comprising anelongated wall closed at its ends and forming a sound attenuatingcavity, and throat-forming means operatively interconnecting said cavityand gas-flow passage whereby each of said elements will attenuate thenoise level of the gases moving through said passage, at least one pairof said elements being mounted in the gas stream in opposed relationshipwith the throat-forming means of each of said pair of elements beingdisposed in their opposed end faces and said opposed end faces beingdisposed closely adjacent each other to define a common extension forthe throat-forming means of said pair of elements, and means supportingeach of saidelements in said gas-flow passage with at leastsubstantially all its wall in spaced relation to said pipe whereby saidgas stream will have to pass between said silencing elements and thepipe during its movement through the conduit.

'11. In a sound attenuating gas conduit for conveying, and attenuatingthe noise level of, a moving gas stream, an open-ended elongated pipeforming a gas-fiow passage wherein a sound source will produce standingwaves having pluralities of harmonic frequencies, and a plurality ofsilencing elements mounted within said pipe along the length thereofwith their adjacent ends in axially spaced relationship, said pipe beingfree from substantial, abrupt changes in its diameter adjacent saidsilencing elements, each of said silencing elements comprising meansdisposed in said gas-flow passage forming a closed resonator volume,said means being positioned in said gas-flow passage with at least amajor portion of said volume spaced from said pipe whereby said gasstream must pass between said means and the pipe during its movementthrough the gas-flow passage, and means on said first mentioned meansforming a resonator volume throat operatively interconnecting saidvolume and gasflow passage whereby said plurality of silencing elementswill be acoustically coupled to said gas stream at points along thelength of said conduit to attenuate the noise level of the gas streammoving through the conduit, one or more of said silencing elementshaving a volume and throat construction to cause said silencing elementto have a frequency response according to the formula 2h+(mean radius ofthe throat cross section) and h is the length of the throat.

12. The invention as set forth in claim 11in which each of said one ormore silencing elements has a throat formed from an elongated tubularelement, and each :of said one or more silencing elements is responsiveto a predetermined sound wave frequency constituting one of the standingharmonic frequencies of the conduit with its associated throatoperatively connected tosaid gas flow passage adjacent a position alongthe length of the conduit where the harmonic frequency for which thesilencing element is responsive reaches its maximum pressure.

13. \An exhaust silencing system for an internal com bustion engine,comprising an open-ended pipe for connection to the engine to receivethe exhaust gases thereof and to convey such gases to a discharge point,said pipe forming a gas conduit wherein the exhaust sound produces oneor more distinct harmonic sound-pressure points at particular locationsalong the length of the conduit equal to fractional lengths thereofaccording to the formula where n is a harmonic number and m is everyinteger between and including 1 and n, and a plurality of silencingelements moiuited in said conduit along the length thereof with theiradjacent ends in axially spaced relationship and in close thermalcoupling relationship with the gas stream, one or more of said silencingelements comprising a resonator volume formed from a closed end pipe andan elongated tubular throat operatively associated with said volume,each of said one or more silencing elements having volume and throatconstruction to cause said silencing element to have a frequencyresponse according to the formula where C is the speed of sound at thetemperature of the gas stream, V is the capacity of the resonatorvolume, and C is the conductivity of the resonator throat equal to2(cross sectional area of the throat) Zh-i-(means radius of the throatcross section) bustion engine, an elongated open-ended pipe constitutinga main gas-flow passage and formed from a plurality of axially extendinginterconnected shorter lengths of pipe, said pipe being free fromsubstantial abrupt changes in its diameter and being adapted to beconnected to said engine for transferring the exhaust gases therefrom,and a plurality of axially spaced siliencing elements mounted withinsaid elongated pipe along the length thereof and having at least themajor portion of their outwardly presented surfaces in spaced relationto said elongated pipe whereby gases moving through said conduit mustpass through the spaces between said silencing elements and saidelongated pipe, each of said silencing elements comprising a length oftubing forming a sound attenuating cavity, and throat-forming means indirect communication with said gas-flow passage operativelyinterconnecting each cavity to the gas-flow passage, 3. first set ofsaid silencing elements having hollow sound attenuating cavities and asecond set of said silencing elements having a porous fibrous soundattenuating material carried in their sound attenuating cavities.

15. The invention as set forth in claim 14 in which the throat-formingmeans in each of the silencing elements in said second set comprises aplurality of perforations formed in said tubing to render the same atleast 30% open, and all of said second set of silencing elements aremounted in said pipe downstream of said first set of silencing elements.

References Cited in the file of this patent UNITED STATES PATENTS1,811,762 Schnell June 23, 1931 1,847,830 Hills Mar. 1, 1932 1,910,672Bourne May 23, 1933 1,947,987 Hathorn Feb. 20, 1934 2,014,666 Peik Sept.17, 1935 2,038,309 Oldberg Apr. 21, 1936 2,056,608 Jack Oct. 6, 19362,099,887 Heath Nov. 23, 1937 2,184,891 Bourne Dec. 26, 1939 2,297,046Bourne Sept. 29, 1942 2,671,523 Walker Mar. 9, 1954 2,694,462 Robbins etal. Nov. 16, 1954 2,748,883 Ralph June 5, 1956 2,795,374 Isakofi June11, 1957 FOREIGN PATENTS 415,446 Great Britain Aug. 27, 1934 678,344Great Britain Sept. 3, 1952 317,630 Switzerland Ian. 15, 1957 559,326Belgium Aug. 14, 1957

1. IN A SOUND ATTENUATING GAS CONDUIT FOR CONVEYING, AND ATTENUATING THENOISE LEVEL OF, A MOVING GAS STREAM, AN OPEN-ENDED PIPE FREE FROMSUBSTANTIAL ABRUPT CHANGES IN ITS DIAMETER FORMING A GAS-FLOW PASSAGEHAVING UNRESTRICTED FLOW AT ITS ENDS, AT LEAST ONE SILENCING ELEMENTMOUNTED WITHIN SAID PIPE INWARDLY FROM THE ENDS THEREOF AND COMPRISINGAN ELONGATED ENCLOSED WALL FORMING A SOUND ATTENUATING CAVITY, ANDTHROAT-FORMING MEANS IN DIRECT COMMUNICATION WITH THE INTERIOR OF SAIDPIPE OPERATIVELY INTERCONNECTING SAID CAVITY AND GAS-FLOW PASSAGEWHEREBY SAID ELEMENT WILL SILENCE THE GASES MOVING THROUGH SAID PASSAGE,THE CROSS-SECTIONAL AREA OF THE GAS PASSAGE WITHIN THE AXIAL EXTENT OFSAID ELEMENT BEING SMALLER THAN THE AVERAGE CROSS-SECTIONAL AREA OF THEGAS PASSAGE OUTSIDE THE AXIAL EXTENT OF SAID ELEMENT, SAID SILENCINGELEMENT BEING MOUNTED WITHIN SAID PIPE IN SAID GAS-FLOW PASSAGE ON ABRACKET SECURED TO SAID PIPE TO SUPPORT SAID SILENCING ELEMENT WHOLLYOUT OF CONTACT WITH SAID PIPE WHEREBY SAID GAS STREAM MUST PASS THROUGHTHE SPACE BETWEEN SAID ELEMENT AND PIPE DURING ITS MOVEMENT THROUGH THECONDUIT.